Assessment
Questionnaire

Introduction to Contagion

Contagion and contagiousness refers to how easily the spread of an infectious disease
is possible
from one person to another.
Other words for contagion include "infection",
"infectiousness",
"communicable", "communicability",
"transmission" or "transmissability".
Common terms used are whether you can "catch" a disease from someone who already has it.

Inheritance and contagiousness unrelated:
Contagiousness has nothing to do with genetics or inheriting diseases from parents.
Genetic inheritance and contagion are not the same thing.
You are born with your genes in your DNA, and they cannot be transferred.
Genetic conditions are inherited from parents, and you have them from birth.
You cannot acquire a genetic condition after birth.
Contagiousness refers to the transfer of an infectious agent (e.g. virus or bacteria)
between people.
For an overview of genetics and inheritance,
see Introduction to Genetics.

Diseases that are contagious:
There are many types of diseases that are caused by an infectious agent
and are therefore contagious to other people.
The main types of diseases that are generally regarded as contagious
include:

Prion diseases: The transfer of infectious proteins is possible for this rare type of disease,
but usually only by direct exposure to brain tissue (e.g. by performing brain surgery or autopsy).

Non-contagious diseases:
Many types of disease are non-contagious.
Obviously physical conditions like an ankle sprain during sports is not contagious.
Here are some other examples of conditions not normally
regarded as contagious:

Genetic diseases: You are born with your DNA and cannot catch bad DNA
from another person.

Cancers: You cannot catch cancer from someone who has it.
It is not contagious by saliva, sexual activity, body fluids, or even by blood.
The only extremely rare ways to catch cancer include organ transplants
and very rare cases of fetus-to-mother or mother-to-fetus contagion.

Autoimmune diseases:
Transmission of autoimmune diseases does not occur from person to person.
Although these diseases are believed to be caused by white blood cells in the blood,
autoimmune diseases are not even believed contagious by blood or body fluids
(presumably a healthy person's immune system simply absorbs and overwhelms
the misbehaving new cells).
The only examples of contagion of autoimmune diseases
are laboratory transfers by immune white blood cell transfer in immunosuppressed mice,
and rare mother-to-fetus contagion of some autoimmune diseases
such as lupus (see neonatal lupus) and myasthenia gravis.
The situation is not entirely clear
with organ transplants,
since some immune system factors can occur,
such as Graft-versus-Host disease.

Mental conditions:
You cannot catch depression, anxiety disorders or other mental conditions
from other people.
Perhaps you can be affected by talking to people, and how they react,
but mental diseases are not infectious.

Hormonal conditions: The inner workings of the body's hormones can have
various disorders, but these are not contagious unless there is an underlying infectious
problem such as a virus actually causing the problem.

Congenital conditions: This term means any condition that you are born with.
All genetic diseases are congenital, but the group of congenital conditions also
includes physical abnormalities that you are born with, such as congenital heart defects.
These conditions are obviously not contagious.

Malnutrition disorders: Obviously you cannot catch a disease that
is caused by a dietary deficiency or general malnutrition.

Chemical or toxic disorders: Obviously a disease caused by exposure
to a chemical, toxin, radiation, or other physical exposure
is not a disease that will spread to others.

Methods of Contagion

There are many ways to catch an infectious disease.
Some are spread by droplets from sneezing,
others by insects,
others by sexual activity,
and
others from blood transfusions or intravenous needle usage.

Droplet infection (including sneezing): Various diseases are spread by droplets
that are inhaled into the respiratory system.
See also diseases contagious from droplet.

Airborne infection: The air can carry droplets, including droplet infection
from sneezing or coughing.
But other types of airborne infection are possible
such as in Legionnaires' disease.
See also airborne diseases.

Fecal-oral contagion: Some conditions, particularly certain worm conditions
are spread by infected feces contaminating the hands, which are then transfered to the mouth.
See also feces and diseases contagious from feces.

Blood contagion: Certain types of disease are spread through blood fluid transfer
from one person to another.
This blood transfer can occur through intravenous needle usage,
blood transfusions (though safe in modern developed countries),
and sometimes by casual blood transfer across wounds.
Organ transplants are also a special case of this type of contagion
with additional issues.
See also diseases contagious from blood,
blood and transplants.

Intravenous needles: The use of intravenous needles for illicit drug use
is one of the most common methods of blood contagion, now that transplants are safer.
See intravenous drug use.

Blood transfusions: Although modern blood transfusions do not transfer disease,
there are still risks in undeveloped areas.
See transfusions.

Blood product transfusions: similar issues exist for transfusions of blood products
rather than whole blood; see transfusions.

Organ transplants: Whether an implanted organ can cause this disease in
the person receiving the organ
if the donor had the disease.
See also transplants.

Fetus-to-mother contagion: extremely rarely, a fetus can get a disease first, and then pass it to the mother through the placenta.

Mother-to-newborn childbirth contagion: certain conditions can be passed to the newborn
during the event of childbirth.
The newborn is exposed to any viruses or bacteria that are present in the vaginal canal,
and some of these can cause disease.
See also childbirth transmission.

Vector-borne contagion: the transfer of the disease by a "vector", usually an insect,
typically from a "reservoir" of the disease, usually in some animal or bird host that is
bitten by the vector insect.

Contagion of a different disease: In a small number of conditions,
a person with one disease is actually likely to spread a slightly different disease.
For example, older people with shingles, which is caused by a delayed recurrence of chicken pox,
can actually spread chicken pox but not shingles itself.
Other types of post-viral syndromes may have similar contagion features.

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